Diode for Alternating Current (DIAC)

1. Theory

• Definition: A DIAC is a two-terminal, bidirectional trigger device with no gate that switches on when the voltage across it reaches a breakover value in either polarity.
• Structure: A symmetric three-layer N-P-N structure with a lightly doped centre layer; both end terminals are equivalent.
• Open-base transistor: In either polarity one junction is reverse biased and the device behaves as a BJT with a floating base.
• Breakover: Avalanche-multiplied leakage at the reverse-biased junction acts as base current and is amplified by transistor gain; the device fires when $M\alpha =1$, the same condition as $B{V}_{CEO}$ of an open-base BJT.
• Symmetry: The symmetric doping keeps the positive and negative breakover voltages nearly equal.
• Placement: It is not a gate-controlled device like the SCR family, but sits in this folder because it is almost always used to trigger a TRIAC.

2. Working Principle

• Blocking state: Below the breakover voltage (typically about 28–36 V) only a small leakage current flows in either direction.
• Breakover: At $V_{BO}$ the current rises sharply through the regenerative avalanche-gain loop.
• Negative-resistance snap-back: After breakover the voltage drops by the breakback voltage $\Delta V$ to a lower on-state value.
• Trigger pulse: A capacitor charged to $V_{BO}$ discharges through the DIAC into the TRIAC gate as a short, sharp current pulse.
• Bidirectional operation: The same behaviour in both half-cycles lets one circuit fire a TRIAC symmetrically on AC.
• Turn-off: The DIAC turns off when its current falls below its holding level, once the capacitor has discharged.

3. Applications

• TRIAC triggering in phase control: Lamp dimmers, universal-motor speed controls and heater controllers.
• Ballast start-up: Starts the self-oscillating half-bridge in compact fluorescent and some LED driver ballasts.

Name	Formula	Related Formulae	Variables/Constants
Breakover condition (open-base transistor)	$M\alpha =1$	$M=\frac{1}{1-(V/B{V}_{CBO}{)}^n}\text{(see Avalanche Diode page)}$	$\begin{array}{r}M:avalanchemultiplicationfactorofthe\\ reverse-biasedjunction\end{array}$ $\begin{array}{r}\alpha :common-basecurrentgainof\\ theopen-basetransistor(seeBJT\\ page)\end{array}$ $n:empiricalmultiplicationexponent$
Breakover voltage	$V_{BO}=B{V}_{CBO}(1-\alpha {)}^{1/n}$	$V_{BO}\approx \frac{B{V}_{CBO}}{{\beta }^{1/n}}\text{(same form as}B{V}_{CEO}\text{on the BJT page)}$	$\begin{array}{r}{V}_{BO}:voltageatwhichthedevice\\ snapsfromtheblockingtothe\\ conductingstate(typicallyabout\\ 28\text{–}36\mathrm{V})\end{array}$ $\begin{array}{r}B{V}_{CBO}:breakdownvoltageofthe\\ reverse-biasedjunctionwiththe\\ otherendopen\end{array}$
Symmetry of the two polarities	$V_{BO}^{+}\approx -V_{BO}^{-},\Delta V_{BO}=\left|V_{BO}^{+}\right|-\left|V_{BO}^{-}\right|$		$\begin{array}{r}{V}_{BO}^{+},V_{BO}^{-}:breakovervoltages\\ forpositiveandnegativepolarity\end{array}$ $\begin{array}{r}\Delta V_{BO}:breakovervoltageasymmetry,\\ keptsmallbythesymmetricdoping\\ profile\end{array}$
Breakback voltage	$\Delta V=V_{BO}-V_{ON}$	$\frac{dV}{dI}<0(I_{BO}<I<I_{ON})$	$\begin{array}{r}\Delta V:dropinvoltageafterbreakover\\ (thenegativeresistancesnapback)\end{array}$ $V_{ON}:conducting(on-state)voltage$ $I_{BO}:breakovercurrent$
Trigger pulse current into a TRIAC gate	$I_p\approx \frac{\Delta V-V_{GT}}{R_g}$	$V_C=V_{BO}\text{at the moment of firing}$	$\begin{array}{r}{I}_p:peakgatepulsecurrentwhenthe\\ capacitordischargesthroughtheDIACand\\ theTRIACgate\end{array}$ $V_{GT}:TRIACgatetriggervoltage$ $\begin{array}{r}{R}_g:seriesresistanceinthedischarge\\ path(neglectingtheDIAC\\ on-resistance)\end{array}$
RC phase-control firing angle	${\alpha }_f=\arctan (\omega RC)+\arcsin \left[\frac{V_{BO}\sqrt{1+(\omega RC{)}^2}}{V_m}\right]$	$\frac{V_m}{\sqrt{1+(\omega RC{)}^2}}>V_{BO}\text{(required for firing)}$	$\begin{array}{r}{\alpha }_f:firingangleoftheTRIAC(see\\ TRIACpage)\end{array}$ $\begin{array}{r}R,C:phase-shiftnetwork\\ resistanceandcapacitance\end{array}$ $V_m:peaksupplyvoltage$ $\omega :angularlinefrequency$ $\begin{array}{r}\text{(ideal}\text{sinusoidal}\text{steady}\text{state;}\text{ignores}\\ \text{charge}\text{left}\text{on}\text{the}\text{capacitor}\text{after}\text{each}\\ \text{pulse)}\end{array}$

Important Questions

Ques: Why does a DIAC have a breakover voltage at all, and why is it described like an open-base transistor?

A DIAC has no gate; both ends are the equivalent of an emitter and a collector with a floating (open) base between them. When a voltage is applied, one junction is reverse biased and sees most of it. As the reverse-biased junction approaches avalanche, its multiplied leakage current becomes base current for the transistor structure, and transistor gain amplifies it. The device snaps to its conducting state when the combined loop reaches $M\alpha =1$, which gives $V_{BO}=B{V}_{CBO}(1-\alpha {)}^{1/n}$: the same relation as $B{V}_{CEO}$ for an open-base BJT. After breakover the voltage falls by $\Delta V$ to the on-state value, which is the negative-resistance snap back used to generate a steep trigger pulse. Reference: S.M. Sze and Kwok K. Ng, Physics of Semiconductor Devices, 3rd ed., Wiley, 2007.

Ques: How does a DIAC trigger a TRIAC, and why is the pulse sharp?

In a phase-control circuit the line voltage charges a capacitor through a resistor. When the capacitor voltage reaches $V_{BO}$, the DIAC breaks over, its resistance collapses, and the capacitor discharges quickly into the TRIAC gate with a peak current of about $(\Delta V-V_{GT})/R_g$. Without the DIAC the capacitor voltage would rise slowly and the gate would be driven gradually near its trigger threshold, giving a poorly defined firing point; the DIAC’s sharp breakover turns this gradual ramp into a short, repeatable pulse, so the firing angle ${\alpha }_f$ of the TRIAC depends mainly on $R$, $C$ and $V_{BO}$. Because the DIAC is symmetric, the same circuit triggers in both half cycles. Reference: General Electric SCR Manual, 6th ed., 1979.

Ques: Why does the symmetry specification matter?

If the positive and negative breakover voltages differ, the TRIAC is fired at different angles in the two half cycles. The resulting asymmetric conduction produces a net DC component in the load current, which can saturate transformers or motors and increases lamp flicker. DIAC datasheets therefore specify a maximum $\Delta V_{BO}$, typically a few volts, and a symmetric doping profile is what keeps it small. Reference: General Electric SCR Manual, 6th ed., 1979.

Related pages in this vault: Triode for Alternating Current (TRIAC), Thyristor (SCR), Bipolar Junction Transistor (BJT), Unijunction Transistor (UJT), Integrated Gate-Commutated Thyristor (IGCT), Bipolar Transistors (BJT and SiGe HBT), Avalanche Diode, Insulated Gate Bipolar Transistor (IGBT), Ovonic Threshold Switch (OTS).